Method for making a curved cube-corner reflector

ABSTRACT

The method of making a curved cube-corner reflector includes making a substantially-flat member having a multiplicity of cubecorner elements therein, and then bending the body against a form having a substantially-smooth curved surface so that the reflector assumes the shape of the curved surface.

United States Patent Heenan et a1.

[ July 10, 1973 METHOD FOR MAKING A CURVED CUBE-CORNER REFLECTORInventors: Sidney A. Heenan, Park Ridge.

Norbert Majewski, Elk Grove Village; Anthony J. Montalbano, Des

Plaines, all of III.

Assignee: Amerace Esna Corporation, Union,

Filed: Sept. 30, 1971 Appl. No.: 185,245

U.S. Cl. 29/416 Int. Cl 823p 17/00 Field of Search 29/416, 400, 155 R,

29/412, DIG. 3

[56] References Cited UNITED STATES PATENTS 2,120,881 6/1938Assbroiche'r et a1 29/416 Primary Examiner-Thomas H. Eager AltorneyCurtis F. Prangley et a1.

[57] ABSTRACT The method of making a curved cube-corner reflectorincludes making a substantially-flat member having a multiplicity ofcube-corner elements therein, and then bending the body against a formhaving a substantiallysmooth curved surface so that the reflectorassumes the shape of the curved surface.

28 Claims, 58 Drawing Figures PAIENIEuJuumm 3744,1117

9m our 10 m9 v FIGJO FIGJS mamtnwu 31441 1? sin 051! 10 FIG. I4

I PAIENIEQJUUO'QIS 5 1744 117 slw user 10 I FIG. 19A

PATENIEDJULIOIBTS 3,744.11?

sum near 1 240 FI6.24 Q FIQZQ PATENTEDJUHOW 3.744.117

am new 10 FIG.37

PAIENIEUJ'" MW r 3744.117

ml 10 U 10 FIG. 45

METHOD FOR MAKING A CURVED CUBE-CORNER REFLECTOR An important object isto provide a method for making a cube-corner reflector in which the axesare normal to the imaginary curved surface defined by the apexes of thecube-corner elements.

Another object is to provide a method for making a part-sphericalreflector having a multiplicity of cubecorner elements therein, the axesof which are radially directed.

There is provided a method of making'a mold for a reflector havingcube-corner elements therein, the method comprising making a plate-likebody having a multiplicity-of elements therein, each of the elementshaving three mutually-perpendicular faces which define a cube corner,providing a form having a substantially-smooth curved surface of thedesired final shape of the reflector, bending the body against the form,so that the body essentially assumes the shape of the curved surface,rigidifying the body to enable it to retain the curvature thereof.

In a preferred form, the method of making the mold comprises the stepsof providing a die having a multiplicity of cube-corner cavitiesrespectively with apexes lying substantially in a plane, electroformingagainst the die to provide a metal member having a multiplicity ofcube-corner projections therein, removing the member from the die,providing a form having a form surface of a selected curvature, cuttingthe form into a plurality of segments each having a portion of the formsurface, cutting the member into a plurality of pieces respectivelyhaving shapes of the form surface portions, bending the piecesrespectively onto the associated form surface portions such that theprojections are directed outwardly, securing the pieces respectively tothe associated form surface portions, securing the segments together, sothat the pieces present a multiplicity of cube-corner projectionsrespectively with apexes defining a surface corresponding substantiallyto the form surface, electroforming against the last-mentionedcube-corner projections to provide a mold having a multiplicity ofcube-corner cavities therein respectively with apexes defining a surfacecorresponding to the form surface.

With the foregoing and other objects in view which will appear as thedescription proceeds, the invention consists of certain novel steps andcertain features of construction, and a combination of parts hereinafterfully described, illustrated in the accompanying drawings, andparticularly pointed out in the appended claims, it being understoodthat various changes in the steps and in the form, proportion, size, andminor details of the structure may be made without departing from thespirit or sacrificing any of the advantages of the invention.

For the purpose of facilitating an understanding of the invention, thereis illustrated in the accompanying drawings preferred embodimentsthereof, from an inspection of which, when considered in connection withthe following description, the invention, its mode of construction,assembly and operation, and many of its advantages should be readilyunderstood and appreciated.

FIG. 1 is a schemative view of a shore line, with a marine lanternmounted adjacent thereto, which marine lantern incorporates therein thevarious features of the present invention;

FIG. 2 is an elevational view of the marine lantern illustrated in FIG.1, on an enlarged scale;

FIG. 3 is a view in vertical section of the marine lantern in FIG. 2, onan enlarged scale, taken along the line 3-3;

FIG. 4 is a view in horizontal section of the marine lantern in FIG. 3,taken along the'line 44 thereof;

FIG. 5 is a fragmentary view, on an enlarged scale, taken along the line55 of FIG. 4, and illustrating one of the sighting markers;

FIG. 6 is a fragmentary view, on an enlarged scale taken along the line6-6 of FIG. 4, and illustrating one of the sighting windows;

FIG. 7 is a sectional view, taken through the sighting marker of FIG. 5,along the line 7-7 thereof;

FIG. 8 is a sectional view, taken through the sighting window of FIG. 6,along the line 8-8 thereof;

FIG. 9 is a perspective view of the cube-corner reflector used in thelantern of FIGS. 1-8;

FIG. 10 is a rear elevational view of the reflector;

FIG. 11 is a view in vertical section of the reflector, taken along theline 1111 of FIG. 10;

FIG. 12 is a view in horizontal cross section, taken along the line12-12 of FIG. 10.

FIG. 13 is a fragmentary view, on an enlarged scale, of the portion ofFIG. 10 within the circle labeled l3, and showing a plurality ofcube-corner reflecting units;

FIG. 14 illustrates one of the reflecting units of FIG. 13 on anenlarged scale;

FIG. 15 is a view in vertical section on an enlarged scale, taken alongthe line 15-15 of FIG. 13;

FIG. 16 is a representation of the manner in which a single cube-cornerelement retrodirectively returns light;

FIG. 17 is a schematic sectional 'view showing the fresnel lens and thereflectorand illustrating how the light source is positioned withrespect thereto;

FIG. 18 is a horizontal section, taken along the line 18-18 of FIG. 17;

FIG. 19 is a schematic view of the collimating lens and cube-cornerreflector, illustrating the manner in which these two elements redirectlight from the source thereof;

FIG. 19A is a horizontal section taken along the line 19Al9A of FIG. 19;

FIG. 20 illustrates a graph comparing the light intensity at variousangular positions with and without the cube-corner reflector;

FIG. 21 is a view showing use of three reflectors mounted on the fresnellens;

FIG. 22A schematically depicts the candlepower distribution of a marinelantern without the reflector incorporating the features of the instantinvention;

FIG. 22B schematically depicts a candlepower distribution of a marinelantern utilizing one reflector which incorporates the features of thepresent invention;

FIG. 22C schematically depicts a candlepower distribution of a marinelantern utilizing two reflectors each of which incorporates the featuresof the present invention;

FIG. 22D schematically depicts a candlepower distribution of a marinelantern utilizing three reflectors each of which incorporates thefeatures of the present invention;

FIG. 23 illustrates a block of material cut into segments;

FIG. 24 illustrates a segmented block with a part spherical surfaceformed thereon;

FIG. 25 illustrates one of the segments of FIG. 24;

FIG. 26 illustrates a die having a multiplicity of cubecorner cavities;

FIG. 27 illustrates, on' an enlarged scale, the portion of FIG. 26within the circle labeled 27, illustrating some of the cavities;

FIG. 28 illustrates a bundle of four pins each having a square outlineand a cube-comer element on the end;

FIG. 29 illustrates the die having a thin electroforming thereon, theelectroforming having been cut away for purposes of illustration;

FIG. 30 is a sectional view, on an enlarged scale, taken along the line30-30 of FIG. 29;

FIG. 31 is a view, on an enlarged scale, of the portion of FIG. 30within the circle labeled 31;

FIG. 32 is a view of the electroform of the die and an epoxy backing onthe electroform;

FIG. 33 is a view in vertical section, on an enlarged scale, taken alongthe line 3333 of FIG. 32;

FIG. 34 illustrates one of the segments shown in FIG. 24, with a pieceof tape having been applied to the partspherical surface thereof;

FIG 35 illustrates the backed up electroform member with a number ofpieces of tape thereon;

FIG. 36 illustrates a piece of the backed up electroformed member afterit has been cut out along the tape margin;

FIG. 37 illustrates the manner in which the electrofomed member isremoved from the epoxy backing;

FIG. 38 illustrates the electroformed member being bent to conform tothe curved surface of one segment;

FIG. 39 illustrates a pressure block being used to secure theelectroformed member to the segment;

FIG. 40 illustrates the segments secured together, respectively havingthe associated electroformed members secured thereto;

FIG. 41 illustrates a mold made by electroforming against the outersurface of the article shown in FIG. 40;

FIG. 42 illustrates a die assembly utilizing the mold of FIG. 41 toproduce molded parts;

FIG. 43 illustrates two reflector sections molded in the mold of FIG.42;

FIG. 44 illustrates the two sections secured together to provide aunitary reflector;

FIG. 45 illustrates the first step in an altemative process of making acurved cube-corner reflector, and depicts a cube-corner mold havingthereon a portion of a plastic part molded thereon;

FIG. 46 illustrates that a portion of the smooth rear surface of thereflector has been removed;

FIG. 47 illustrates a number of pieces of tape on the molded reflector;

FIG. 48 illustrates a cutout from the reflector 47 being bent to theshape of the curved surface of one of the segments;

FIG. 49 illustrates the segments secured together respectively havingthe associated molded members secured thereto;

FIG. 50 illustrates a mold made by electroforming against the outersurface of the article shown in FIG. 49;

FIG. 51 illustrates a step in a process by which a partcylindricalreflector may be made, and depicts a portion of a cylinder; 7

FIG. 52 illustrates a thin electroformed member bent to conform to theshape of the outer surface of the cylinder of FIG. 51;

FIG. 53 illustrates a mold made by electroforming against the outersurface of the article shown in FIG. 52; and

FIG. 54 illustrates a part-cylindrical reflector molded against the moldshown in FIG. 53.

Turning now to the drawings, and more particularly to FIG. 1 thereof,the details of the instant invention will be described. There is shownin FIG. 1 a marine lantern which is positioned on the shore 101 adjacenta body of water 102, the shore line 103 between the shore and the waterbeing irregular as indicated. The purpose of the marine lantern 100 inthis particular installation is to establish a reference point on theshore. Thus, light that is directed over the shore 101 would be uselessin accomplishing this objective. The marine lantern 100 includes means,to be described in detail hereinafter, which reduce the amount of lightdirected onto the shore 101 and maximize the amount of light 104directed over the water 102.

Turning now to FIGS. 2 and 4, the details of construction of the marinelantern 100 will be described. The marine lantern 100 is mounted on afoundation 105 which is in place on the shore 101. A power cable 106provides power to'the light source located within the lantern 100.

The marine lantern 100 comprises a base 110, which base 110 includes aset of four outwardly-directed feet 111 spaced 90 from each other. Inthe outer end of each foot 111 is an opening 112 through which is passeda bolt 113 into the foundation 105 so as to secure the marine lantern100 thereto. The base 110 further includes a wall 114 which flaresupwardly and outwardly. Near the upper end of the flared wall 114 atdiametrically-opposite points threon is a pair of upwardly-directedstuds 115 and associated wing nuts 116. The studs 115 and the wing nuts116 secure a bracket 117 in the base 110, which bracket 117 may carry aflasher assembly 118. The flasher assembly 118 is shown in phantom,since this is a well-known mechanism and may or may not be utilized,depending upon whether it is desired to provide a flashing light or acontinuous light. Projecting outwardly from the top of the wall 114 aretwo sets of spaced-apart ears 119.

The marine lantern 100 further comprise a frame member 120 which isgenerally found in outline, a pair of la'terally-spaced-apart ears 121being directed outwardly and respectively into the spaces between thetwo sets of ears 1 19. A bolt 122 and a nut 123 pivotally mount each ear119 and the associated pair of ears 121, whereby to pivotally mounttheframe member 120 onto the base 110. The frame member 120-can be pivotedcounterclockwise, as shown in FIG. 3, to expose the interior of the base110 to gain access to the flasher assembly 118. There is also providedthree screws 123a which respectively threadably engage the outermostportion of the flared wall 114 and the frame member 120, thereby tosecure the frame member 120 to the base 110. In order to pivot the framemember 120, the

screws 123a would have to be removed. The frame member 120 also carriesa level 124 which permits the installer of the marine lantern 100 toadjust the mounting therefor, so that the lantern 100 is perfectlylevel.

The frame member 120 includes three arms 125, each directed upwardly andinwardly and terminating in a substantially-horizontal portion 126. Inorder to mount a bracket 127, there is provided three threaded bolts 128respectively passing downwardly through the horizontal portions 126.Each bolt 128 has a head 129 and passes through a compression spring130, through an associated opening in the bracket 127, through a bushing131, and threadably engaging a wing nut 132. The bracket 127 is adaptedto carry a bulb-changer mechanism 133. The bulb-changer mechanism 133 isshown in phantom, since it may be of standard construction and does notform part of the instant invention. The bulb-changer mechanism 133operates sequentially to replace a burnt-out bulb with a fresh bulb. Oneof the sockets 134 is shown carrying a bulb 135, which bulb 135 has afilament 136. The wing nuts are used to accurately position the filament136 in a manner to be explained hereinafter. For the present, it shouldbe noted that tightening the wing nuts 132 brings the filament 136upwardly, and loosening them causes the filament 136 to move downwardlyby virtue of the force exerted by the springs 130. Of course, byselectively adjusting the wing nuts 132, the filament 136 can be tilted,in addition to being moved upwardly and downwardly. Also, there isprovided a slightlylarger opening in each horizontal portion 126, toenable lateral movement of the filament 136.

The marine lantern 100 further comprises a fresnel lens 140 in the shapeof the frustum of a cone, which, in the embodiment shown, has a veryslight deviation from a perfect cylinder. The purpose of inclining thewall defining the lens 140 is to provide the proper draft angle toenable the part to be withdrawable from the mold therefor. The interiorsurface 141 of the lens 140 is also frustoconical in shape and issubstantially smooth. Formed on the outer surface of the lens 140 is aplurality of dioptric rings 142 extending from an upper end 143 to alower end 144, which dioptric rings are of basically known construction,whereby the details of construction thereof will not be described.

There is provided a central dioptric ring 142a, a number of lowerdioptric rings 142b between the central dioptric ring 142a and the lowerend 144, and a lesser number of upper dioptric rings l42c between thecentral dioptric ring 142a and the upper end 143. The focal point of thedioptric rings 142 is located in a plane passing through the center ofthe central dioptric ring 142a, at the point of intersection with theconical axis of the lens 140. The upper end 143 terminates in a lip 154are used accurately to position the filament 136 of the bulb 135, aswill be explained hereinafter.

The marine lantern 100 further comprises a cover 160 which isconstructed of transparent material and includes a frustoconical sidewall 161 having substantially smooth inner and outer surfaces. The topof the side wall 161 merges into a convex top wall 162, and the bottomof the side wall 161 merges into a flange 163 that is outwardlydirected. Formed in the inside of the flange 163 is an annular recessdefining a shoulder 164.

The cover 160 is slightly wider than the lens 140, but has the samegeneral slope to the side walls, whereby, when the cover 160 is inplace, there is a space between the dioptric elements 142 and the cover160. The shoulder 164 on the flange 163 rests against the upper surfaceof the flange 146 on the lens 140. There is provided an annular clampingring 170 having an offset portion 171 bearing against the upper surfaceof the flange 163 and is secured to the frame member by means of aplurality of screws 172.

The lantern 100 further comprises a reflector 190. Turning now to FIGS.9 to 12, the details of the reflector 190 will be described. Thereflector 190 has a front surface 191 which is substantially smooth andis part spherical in form with a given center of curvature. Thereflector 190 has a pair of side edges 192 and 193 respectively lying inplanes containing the center of curvature of the front surface 191. Thereflector 190 further has a pair of end edges 194 and 195 respectivelylying in chordal planes which are disposed parallel to each other and donot pass through the center of the curvature of the front surface 191.For reasons to be explained hereinafter, the planes which contain theside edges 192 and 193 are not perpendicular to the planes which containthe end edges 194 and 195, and, accordingly, the reflector 190 isslightly skewed. 1f desired, narrow flanges 196 and 197 may be formedrespectively on the end edges 194 and 195 to facilitate stripping thereflector 190 from the mold. Formed in the reflector 190 is a pair oflaterally-spaced-apart mounting holes 198 midway between the end edges194 and 195. Disposed midway between the mounting holes 198 is asighting hole 199. There is also provided a pair of fingers 200 and 201directed rearwardly and located on a line passing through the sightinghole 199 and perpendicular to the line joining the mounting holes 198,the fingers 200 and 201 being equidistant from the sighting hole 199.For reasons to-be explained subsequently, the finger 200 is aboutone-half the length of the finger 201.

that is directed inwardly and slightly upwardly, and

the lower end 144 terminatesin a flange 146 directed outwardly. As' isbest seen in FIGS. 3 to 8, there is provided in the central dioptricring 142a four bosses 147, 148, 149 and respectively spaced at 90intervals. Thus, the boss 147 is diametrically opposite the boss 149 andthe boss 148 is diametrically opposite the boss 150, all located in thecentral dioptric element 142a. Formed in the boss 147 is a window 151,in the boss 148 a window 152, in the boss 149 an X-shaped mark 153, andin the boss 150 an X-shaped mark 154. The outer surface of the windows151 and 152 are curved to match the curvature of the interior surface141 so that light therethrough is substantially undeviated. The windows151 and 152 and the X-shaped marks 153 and The reflector is divided intoa pair of symmetric sections 205a and 205b which are substantiallyidentical. The ensuing remarks will be directed to the section 205a butit is to be understood the remarks are equally pertinent in respect tothe section 205b. The section 205a has side edges which respectively liein planes containing the center of curvature of the sections frontsurface and has a longer end edge 206a which also lies in a planecontaining the center of curvature of the front surface 191, the planecontaining the edge 206aand the plane containing the end ege 194 being Isubstantially parallel. The surface 205a is divided into a plurality ofzones 207a separated by boundary lines 208a. The planes containing theboundary lines 208a are disposed parallel to one another and do not passthrough the center of curvature of the front surface 191. The end edges206a and 206b are welded together to provide the unitary reflector 190depicted. The side edges of the section 205a merge and are continuationsof the corresponding side edges of the section 205b, thereby to providethe side edges 192 and 193.

In the form shown, the angular extent of the reflector 190 measuredbetween the side edges 192 and 193 is about 60, and the angular extentof the reflector 190 between the end edges 194 and 195 is about 110.Since the side edges 192 and 193 of the reflector 190 lie in planescontaining the center of curvature of front surface 191, two or moresuch reflectors may be butted together, side by side, so as to increasethe angular extent from side edge to side edge. Further details on thisembellishment will be described hereinafter.

Formed in the rear surface of the reflector 190 is a plurality ofjuxtaposed reflector units 210, the details ary forming the outline ofthe element 211. The other intersections between adjacent faces 215extend from the apex 216 to a point at the side of the square boundaryof the element 211. The faces 215 extend substantially at right anglesto each other to form a portion of a cube, and, hence, the reflectorelements will hereinafter be referred to as cube-corner elements. It isto be understood, however, that a'cube-corner element is not limited toone having faces of equal area but solely indicates that three faces areperpendicular to one another. I The point 217 constitutes the center ofthe reflecting unit 210. The element 211a is rotated 90 with respect tothe element 211b, the element 211b is rotated 90 with respect to theelement 2110, the element 21lc is rotated 90 with respect to element211d, and the element 211d is rotated 90 with respect to the element211a. Each reflecting unit 211 provides four interior cube corners, theadjacent elements of which are oriented 90 relative to each other. Eachreflector element, such as, for example, the reflector element 211, hasa cube axis 219 (see also FIG. 16), which cube axis is an imaginary linedrawn through the apex 216 with respect to which the three faces 215 aresymmetrically arranged and with'respect towhich the three edges 2141 aresymmetrically arranged. Thus, each face 215 forms an angle of 35 16"with the cube axis, and each edge 214 forms an angle of 54 44 with thecube axis.

As is best seen in FIG. 15, the apexes 216 of the reflector elements 211define an imaginary surface 218 which is substantially part sphericaland substantially parallel to the front surface 191 of the reflector190. The cube axis of each reflector element 211 is substantiallyperpendicular to the tangent to the imaginary surface 218 at the apex ofthat reflector element 21 1. The imaginary surface 218 in any given zone207a is curved in the direction of elongation so as to be parallel tothe associated portion of the front surface 191. However, for reasons tobe explained hereinafter, the imaginary surface 218 in the directionnormal to the direction of elongation is substantially flat. Thus, byincreasing the number of zones 207a, the better the entire imaginarysurface 218 approximates the surface ofa sphere. However, the greaterthe number of zones 2070, the more boundary lines 208a, which boundarylines 208a adversely affect the immediately-adjacent reflector elements211.

It should be apparent that with this type of construction the cube axesof the reflector elements 211 are all substantially radially directed,that is, aligned with the center of curvature of the front surface 191and substantially with the center of the center of curvature of theimaginary surface 218. Light emanating from a light source positioned atthat center of curvature will impinge the reflector elements and will beretrodirectively returned by the reflector elements 211 back to thesource. This mode of operation is particularly shown in FIG. 16 whereina filament 136 constituting a light source emits a ray oflight 136adirected parallel to the cube axis 219 of the reflector element 211,which ray strikes one of the faces 215 of that reflector element 211. Ina known way, the light ray 136a strikes the other faces 215 of thatreflector element 211 and is returned as a light ray 13612 disposedparallel to the ray 136a and to the cube axis 219. Although the light isshown returning to a point spaced from the filament 136, it should beunderstood that the reflector element 211, in practice, is very tiny, onthe order of 0.040 inch, so that the maximum displacement of thereturned light from the source would be 0.040 inch.

If the filament 136 is displaced downwardly so as to emit a ray of light1360 which is not parallel to the cube axis 219, but is rather at anacute angle with respect thereto, the reflector element 211 redirectsthe ray 1360 so as to return as a ray 136d, again parallel to theincoming ray 136c but displaced therefrom by a maximum of 0.040 inch. Aslong as the angle formed by the incoming ray 136C and the cube axis 219is less than a critical angle, the reflected ray 136d will returnsubstantially to the source. In a cube-corner reflector, in which thefront surface is smooth and a cube corner projects from the rearsurface, and is composed of methyl methacrylate resin, this criticalangle is about l9. Thus, as long as the incoming ray forms an angle ofil9 with respect to the cube axis 219, the light will be returnedsubstantially to the source thereof. This feature, as will be explainedfurther, facilitates placement of the bulb135. Specifically, if thefilament 136 is precisely at the center of curvature of the reflector190, all the light rays which impinge on the reflector 190 will beretrodirectively returned to the filament 136, since the cube axes 219are radially directed. However, even if the filament 136 is slightlyremoved from the center of curvature, the light will still be returnedby the reflector'190 to the filament 136 by virtue of the phenomenondescribed with respect to FIG. 16.

Referring now to FIGS. 3 and 4, the manner in which the reflector 190 ismounted will be described. A pair of openings is drilledin the centraldioptric ring 142a respectively on either side of the boss 148. Thereflector 190 is then placedin position as shown such that the outerends of the fingers 200 and 201 engage the interior surface 141 of thelens 140. The finger 200 is shorter than the finger 201, as the resultof the lens having a frustoconical shape. A pair of headed studs 220 isinserted through the holes just formed inthe central dioptric ring 142aand through the mounting holes 198 in the reflector 190. A pair ofpush-on fasteners 221 is applied respectively to the headed studs 220 tosecure the reflector 190 in place. In this condition, the sighting hole199 in the reflector 190 will be aligned with the window 152.

The manner in which the light bulb 135 is positioned in the marinelatern 100 will be described. Referring first to FIGS. 3 and 4, theinstaller removes the screws 123a to permit the frame member 120 to bepivoted and expose the wing nuts 132. The installer looks through thewindow 152 which is aligned with the sighting hole 199 in the reflector190, the X-shaped mark 154 being diametrically opposite the window 152.The installer looks through the window 152 and adjusts the wing nuts 132to raise or lower the changer mechanism 133 until the filament 136 ofthe bulb 135 is aligned with the X-shaped mark 154. The changermechanism 133 can be moved laterally to enable lateral alignment of thefilament 136. Then, the installer looks through the window 151 andreadjusts the wing nuts 132 until the filament 136 is in alignment withthe X-shaped mark 153. Since this latter adjustment may have affectedthe initial adjustment, the installer then looks through the window 152and so on until the filament 136 is aligned in both directions. Whenthat occurs, the filament 136 is precisely at the intersection of theconical axis of the lens 140 and the plane passing through the centraldioptric ring 142a. It is important that the filament 136 be accuratelyplaced in respect to the dioptric rings 142, which is accomplished asabove explained. If the filament 136 is not located at the focal pointof the dioptric rings 142, the light will not be properly collimated bythe lens 140.

As previously explained, the reflector 190 is so designed that itscenter of curvature will fall generally at the focal point of thedioptric rings 142. Thus, positioning the filament 136 accurately, asabove described with reference to FIGS. 17 and 18, to place it at thefocal point of the dioptric rings 142, results in the filament 136 beingalso approximately at the center of curvature of the reflector 190.However, the placement of the filament 136 with respect to the reflector190 is not critical, by virtue of the ability of a cube-corner reflectorelement to return light to its source within an angle of as much as il9.It should be understood that, if the reflector 190 had a simplespherical surface, the filament 136 would have to be placed accuratelywith respect to both the dioptric rings 142 and the reflector 190, analmost impossible task, since there is only one degree of freedom tosatisfy two parameters.

Referring now to FIGS. 19 and 19A, the details as to the manner in whichthe marine lantern 100 functions will be described. Light rays l36e fromthe filament 136 strike the reflector 190, and, by virtue of itsretrodirective reflecting capabilities, those light rays are returnedback to the filament 136, so as to reinforce the light therefrom. Thelight rays 1362 being emitted directly by the filament 136 and/orreinforced by light reflected by the reflector 190 strike the interiorsurface 141 of the lens 140 and are refracted and then refracted againwhen they strike the dioptric rings 142, so as to emerge as light rays136 f disposed horizontally. Thus, the lens 140 serves to condense in atleast one direction (that is, vertically in the form shown) the lightemitted by the filament 136. Although only light rays 136fare shown inFIG. 20, it is to be understood that the lens 140 emits a solid beam oflight extending from the uppermost light ray l36fto the lowermost lightray 136]". If the angular extent of the reflector 190-measured in thehorizontal plane is 60, then the angular ex-. tent of the beam would beabout 300, there being no light emitted in the region of the reflector190. Thus, in the region where no light is needed, light is not emitted,but, rather, is used to reinforce and intensify the light emitted in theuseful region. Accordingly, the wattage of the bulb may be less thanwhat would be needed if the reflector were not used, or the same wattagebulb could be used and thereby increase the intensity of the lightwithin the desired region.

The angular extent of the reflector 190 in the vertical direction (inthe direction of its elongation) is such that a light ray 136e from thefilament 136 that strikes the upper end edge 194 will beretrodirectively reflected back to the source and thence to thelowermost dioptric ring 142. Stated in another way, a point on the upperend edge 194 of the reflector 190, the filament 136, and a point on thelowermost dioptric ring 142 should lie in a straight line. Similarly, apoint on the lower end edge 195 of the reflector 190, the filament 136,and a point on the uppermost dioptric ring 142 should lie in a straightline. If the angular extent of the reflector 190 were any greater, thelight rays striking the extremities of the reflector 190 would beretrodirectively reflected back through the filament 136 and then alonga path which does not intersect the lens 140.

In order that each point on the upper end edge 194 of the reflector 190have a corresponding point on the lowermost dioptric rings 142, theupper end edge 194 should lie in a plane parallel to the horizontalplanes which define the upper and lower boundaries dioptric rings 142.Similarly, the lower end edge 195 of the reflector 190 should lie in ahorizontal plane parallel to the planes defining the dioptric rings 142.

It should be pointed out that if the light bulb 135 is sufficientlyclose to the lens 140, some of the dioptric rings would have to be ofthe catadioptric type in which both reflection and refraction occurs tobend the rays sufficiently.

Referring to FIG. 20, which plots angular position as the abscissa andlight level readings as the ordinate, the curve 230 represents thecondition when the reflector 190 is masked. The curve 230 deviates froma theoretically flat curve due to shielding by the bulb support andnonuniform output by the bulb 135. The curve 231 represents thecondition with the reflector 190 unmasked. Within approximately the 60opposite the 60 range of the reflector 190, there is substantialimprovement in light output. It should be understood that the ordinatedoes not represent actual light levels but rather relative meterreadings. Thus there is about a 30 percent improvement in lightintensity within the 60 opposite the 60 range of the reflector 190.

The dip in the curve 231 at 0 is due to the discontinuities in thereflector 190 caused by the sighting hole 199 and the fingers 200 and201.

The marine lantern 100 is constructed to be usable in an environmentsuch as shown in FIG. 1, that is, on a piece of land that juts into thewater and only a narrow angular region does not require light. If theregion not requiring light were expanded, additional reflectors 190 canbe utilized. Reference is made to FIG. 21 which illustrates threereflectors 190 arranged side by side and all mounted to the lens 140. Inthis case, light rays 136k from the filament 136 directed toward theleft, as viewed in FIG. 21, will strike one of the reflectors 190 andretrodirectively will be returned to the filament 136. Light emanatingdirectly from the filament 136 or being retrodirectively returnedthereto is emanated in the form of rays 136g. Thus, light which isotherwise wasted is returned to the filament 136, to reinforce the lighttherefrom that is directed to the desired region.

In lining up the bulb 135 when three reflectors are mounted in themarine lantern 100, the middle reflector 190 will be in the sameposition as that shown for the single-reflector embodiment of FIGS. 1 to20. Accordingly, the window 152 is aligned with the hole 199 in themiddle reflector 190, thereby permitting to enable viewing of theX-shaped mark 154. The window 151 is disposed immediately to the rightof the lowermost reflector as viewed in FIG. 21, and the X-shaped mark153 is immediately to the right of the uppermost reflector 190 as viewedin FIG. 21 Accordingly, there is provided a line of vision between thewindow 151 and the X-shaped mark 153 to permit the requisite adjustmentof the bulb 135.

It is to be understood that two reflectors 190 may be utilized and itwould, of course, depend upon the specific need as to whether one, two,or three reflectors 190 would be required. With no reflectors, the lightdistribution will be as shown in FIG. 22A. With one reflector a 60 zonewill be blacked out and the light projected into the opposite 60 zonewill be intensified (see FIG. 228). With two reflectors, a 120 zone isblacked out and the light projected into the opposite 120 zone isintensified (see FIG. 22C). With three reflectors, a 180 zone is blackedout and the light projected into the remaining 180 zone is intensified(see FIG. 22D). Of course, whatever zone is blacked out will appear darkso as not to be a nuisance to persons living in the area near the marinelantern. Also, by preventing light from being projected into anunnecessary zone and utilizing that light in another zone, a lesserwattage light bulb may be utilized and still achieve the same lightintensity, at least in a portion of the zone. Of course, a lesserwattage light bulb consumes less power and therefore costs less tooperate. If a battery is used, this would mean an increased life for thebattery.

As was previously explained, the planes respectively containing the sideedges 192 and 193 of the reflector 190 are not perpendicular to theplane passing through the holes 198 and 199, that is, a horizontalplane. Thus, when the reflector 190 is mounted, such that the holes 198and 199 are horizontal, the side edges 192 and 193 are skewed, that is,the planes containing these side edges are not arranged vertically. Whentwo or more reflectors 190 are butted together, the side edge 192 of oneof the reflectors will abut and perfectly match the side edge 193 of theother reflector 190. In this way, the juncture between the adjacentreflectors 190 is not disposed in a vertical plane. If the line were ina vertical plane, a viewer, standing at a point on a line passingthrough the filament 136 and that vertical line, would see a substantialdecrease in light intensity over that which he would see on either sideof that point.

The zones 207a and 207b are horizontally arranged for similar reasons.If the zones were vertically arranged, then the boundaries 208a and2081) would be disposed in vertical planes, and there would be a numberof angular positions at which the light intensity would be substantiallyreduced. By arranging these zones 207a and 207b horizontally, the amountof light at any angular position is only very slightly affected,

since the viewer sees a narrow, vertically-arranged strip I of lightwhich appears vertically continuous by virtue of the action of thedioptric rings 142.

By constructing the edges 194 and 195 such that they are in planesparallel to each other and horizontally disposed when the reflector 190is mounted in use, when two or more reflectors 190 are butted together,the upper and lower edges will continue to be in horizontal planes. Thismeans that each point on the upper edge of the reflectors 190 will havea corresponding point on the lowermost dioptric ring 142. Similarly,each point on the lower edge of the abutting reflectors 190 will have acorresponding point on the uppermost dioptric ring 142.

In practice, the Fersnel lens 140 does not collimate the lightperfectly, first, because the dioptric rings 142 are not perfect information and, second, because the filament 136 is not a point source oflight. Rather, it has length so that a ray of light emitted from oneportion of the filament would be deviated slightly when it isretrodirectively reflected by the reflector 190. The inaccuracies in thedioptric rings 142 and the fact that the filament 136 is not a perfectpoint source, causes the beam from the marine lantern 100 to spreadvertically slightly.

Although the reflector 190 finds particular use in a marine lantern 100of the type described, it is ,to be understood that such reflector has agreat number of other uses. The reflector may, for example, becylindrical, in which case the imaginary surface defined by the apexesof the cube-corner elements is part cylindrical, and the cube axes wouldbe perpendicular to the imaginary surface. Also, it is to be noted thatalthough the reflector 190 has cube-corner elements of a square outline,elements having a hexagonal outline or a triangular outline arecontemplated, although the square outline is preferred.

Turning now to'FIGS. 23 to 44, the details of the method for making thereflector 190'will be described. Referring specifically to FIG. 23,there is shown a block 240 in the shape of a rectangular parallelepiped.The block. 240 is divided into twelve segments 241, the width of each ofwhich increases from left to right, as viewed in FIG. 23. A rod 242 ispositioned through the segments 241 and another pair of rods withthreaded ends are also positioned through the segments 241, nuts 243being threaded onto the rods. The rods hold the segments 241, as shown.A part-spherical surface 244 is formed on the top surface of the block240, as shown in FIG. 24. Each segment 241 has a pair of sides 245 lyingin parallel planes, whereby the upper surface of each segment 241defines a zone 246. The thickness of the segments 241 varies in orderthat the lateral (measured transverse to the sides 245) arc length of Ieach zone 246 is equal. Thus, a segment 241 with substantial angle willbe thinner than a segment 241 with less angle.

The next step is to form a cube-corner member. To that end, there isprovided a die 250, as shown in FIG. 26, having four side walls 251rectangularly arranged and a bottom wall 252. Mounted within the sidewalls 251 is a part 253 having therein a multiplicity of cubecornerunits 254 (FIG. 27). Each cube-corner unit has four cube-corner cavities255, each being rotated with respect to the adjacent cube-corner cavity.Each cube-corner cavity 255 is comparable to that shown in FIGS. 13through 16, but, however, in the form of a cavity instead of aprojection. To make the part 253, an array of pins are held together.Reference is made to FIG. 28 which illustrates a single pin bundle 257made up of four pins 258, each having a square outline and a cube-cornerprojection 259 at the outer end thereof. Each cube-corner projection 259is made up of three mutually-perpendicular faces 261 having a commonapex 260. When a number of the pin bundles 257 are grouped together,they may be placed in a plating tank in which nickel or the like isdeposited or electroformed onto the cube-corner projections 259. After aperiod of time, a sufficient thickness of material has beenelectroformed onto the cube-corner projections to render the electroformself-supporting. At that time, it is pried off of the pins 258, and theelectroforming that is separated therefrom, after being cut andotherwise treated, becomes the part 253 in the die 250. Of course, thesteps of electroforming are known in the art, whereby the abovedescription is a sketchy one, simply to describe the over-all process.It is to be understood that there may be a great many steps in theprocess of forming the pins into the desired array, all the way up toobtaining a rectangular electroform for use in the die 250.

The exposed surface of the part 253 which has therein the cube-cornercavities 255 is then plated with chrome or other suitable separatingmedium. The die 250 is then inserted into a plating tank, whereby nickelor other plating material is deposited or electroformed onto thechrome-plated surface of the die 250. The plating is carried on for asufficient amount of time to generate a metal member having acrudelyconfigurated rear surface 271 and a precisely configurated frontsurface 272 (FIGS. 29 and 30). In a construction of the presentinvention, the thickness of the metal member 270 was 0.007 inches in theareas of the apexes of the cube-corner elements 255 and 0.004 inches inthe region of the valleys of the cube-corner projections 255. Thisvariant thickness is a characteristic of the electroforming process. Ofcourse, the front surface 272 has therein a multiplicity of cube-cornerprojections, like the pin bundle shown in FIG. 28.

Turning now to FIGS. 32 and 33, an epoxy backing 275 is applied to therear surface 271 of the metal member 270. In order to obtain the desiredshape of the epoxy 275 when it dries, a suitably-shaped form should beprovided. The epoxy 275 assures that the metal member 270 will retainits shape during subsequent steps, particularly prying it off the die250.

Referring to, FIG. 34, the next step involves placing a piece of tape277 on the zone 246 of each segment 241. The tape 277 is then cut so astoconform precisely to the zone 246. The tape is then removed and isapplied to the configurated surface 272 of the epoxybacked metal member270. Five such pieces of tape are shown in FIG. 35 thereby additionalepoxy-backed metal members 270 would have to be provided to cover theother seven segments. The configurated surface 272 is then painted,preferably by spray. The pieces of tape are then removed, and theepoxy-backed metal member 270 is then cut along the boundaries betweenthe painted and unpainted areas. Preferably, each piece 279 (FIG. 36)thus cut out, is slightly larger than the zone 246 of the associatedsegment 241. Each epoxybacked piece 279 is then heated to soften theepoxy 275 and then a tool 281, such as shown in FIG. 37, is

used to separate the piece 279 from its epoxy backing. As depicted inFIG. 38, the piece 279 is then bent onto the zone 246 of the associatedsegment 241. It should be understood that each zone 246 is partspherical and, as a practical matter, it is impossible to bend the piece279 both longitudinally and laterally entirely to conform to the zone246. Instead, the piece 279 is bent only longitudinally to conform tothe zone 246 in that direction, and the piece 279 is left essentiallyflat in the later direction. By constructing the segments 241 to bereasonably narrow, the over-all lateral surface will approximate aspherical curvature;

Referring now to FIG. 39, there is provided a press 282 having a concavesurface 283 to which is secured a rubber strip 284. Adhesive is thenapplied to the rear surface 271 of the piece 279 and to the zone of theassociated segment 241. The press 282 is positioned, as shown in FIG.39, and pressure applied thereto and maintained until the piece 279 issecured. Since the piece 279 was cut over-size, its edges 280 overhangthe associated zone, which edges 280 are then ground, so that the piece279 has precisely the right shape.

A similar process to that depicted in FIGS. 36-39 is performed inrespect to each piece 279 and each segment 241. The segments 241 arethen reassembled, as shown in FIG. 40. The outer surface of each piece279 becomes a continuation of the outer surface of the adjacent piece279, thereby forming a continuous configurated surface 286 having amultiplicity of cube-corner projections thereon.

Turning now to FIG. 40, the exposed surface 286 is covered with chromeor other suitable separating medium. After the unit shown in FIG. 40 issuitably masked, it is inserted in the plating tank to plate a quantityof nickel or other material onto the surface 286. After a suitablethickness of nickel has built up, the unit shown in FIG. 40 may beremoved, the front surface of the built-up part masked and then returnedto the plating bank for back up. The completed mold 290 is shown in FIG.41. The mold 290 has a configurated front surface 291 consisting of amultiplicity of cube-corner cavities. The mold 290 has a pair of sideedges 292 and 293 respectively lying in planes containing the center ofcurvature of the surface 291. The mold 290 further hasa pair of endedges 294 and 295 respectively lying in chordal planes which aredisposed parallel to each other and do not pass through the center ofcurvature of the surface 291. The front surface 291 is divided into aplurality of zones 296, separated by boundary lines 297. The planescontaining the boundary lines 297 are disposed parallel toone anotherand do not pass through the center of curvature of the surface 291. Theboundary lines 297 result from the slight discontinuities from thevarious pieces 279 that make up the surface 286. Of course, each zone296 is spherically curved in the longitudinal direction, but isflattened in the lateral direction, since that is the basic shape of thepieces 279 which make up the surface 286. A hole 298 is formed in themold 290, which hole 298 forms the fingers 200 and 201 in the finishedproduct. Suitable projections may be mounted at the end edge 295 toenable formation of the holes 198 and 199 in the completed product.

Turning now to FIG. 42, the mold member 290 provides the lower half of amold assembly incorporating an upper mold member 300. The mold member300 has a smooth part-spherical surface 302 and a gate 301.

When in position, as shown in FIG. 42, molten acrylic is admitted intothe space between the configurated surface 291 of the mold member 290and the smooth surface 302 on the mold member 300, in the usual manner.After the mold member 300 is separated from the mold member 290, areflector section 205a (FIG. 43) is withdrawn. A second molding cycle isperformed, utilizing the mold assembly shown in FIG. 42 to provide asecond reflector section 205b. The reflector section 205b is an exactduplicate of the section 205a, but is oriented 180 with respect tosection 205a, all as shown in FIG. 42. As previously explained, thesections respectively have longer edges 206a and 206b, both of which liein planes containing the same center of curvature, whereby they may bemated, as shown in FIG. 44, and then welded together. The reflector 190shown in FIG. 44 is a reflector previously described herein, except forthe absence of flanges on the outer end edges 194 and 195. If desired,these ends, which are used for stripping purposes, would be added duringthe molding operation. Each reflector 190 is made in two sections 205aand 205b so as not to exceed the draft angle necessary to withdraw thepart from the mold.

The boundary lines 208a and 208b result from the corresponding boundarylines 297 in the mold member 290. Each zone 207a and 207b will bepart-spherical in the longitudinal direction, but substantiallyflattened laterally since that is the characteristic of the mold member290.

Turning now to FIGS. 45-50, the details of a second method of making acurved reflector will be described. The same die 250 utilized in thefirst process is also used in this alternative process. Acrylic ismolded onto the surface, having the cube-corner cavities 255, to providea reflector 310 having an over-all thickness, for example, of 0.080inch, as shown in the right-hand side of FIG. 46.-The reflector 310 hasa flat rear surface 31 1 and has a configurated front surface 312 whichhas therein a multiplicity of cube-corner projections, much like the pinbundle shown in FIG. 28. The rear surface 311 may then be ground down,if desired, the resulting surface being labeled with a number 313. Ofcourse, FIG. 46 represents the grinding process while it is beingperformed, so that, when completed, the entire rear surface will be at alevel represented by the number 313. The amount of grinding would bethat necessary to render the reflector 310 substantially pliable. If theoverlay (distance between the surface 313 and the lowest point in thesurface 312) were about 0.010 inch, the reflector 310 would besufficiently pliable.

As was done in respect to the first process described, pieces of tape277 are respectively applied to the zones 246 of the segments 241. Afterthe tape 277 is cut to conform precisely to the associated zone 246, itis removed and applied to the configurated surface 312 of the reflector310, as shown in FIG. 47. Five such pieces of tape respectivelyconforming to different ones of the segments 241 are applied to thesurface 312. The conflgurated surface 312 is then painted, and thepiecesof tape are removed. The reflector 310 is then out along the boundariesbetween the painted and unpainted areas. Preferably, each pieces 320(FIG. 48) thus cut out, is slightly larger than the zone 246 of theassociated segment 241. As depicted in FIG. 48, each piece 320 is thenbent onto the zone 246 of the associated seg- .ment 241. It should beunderstood that each zone 246 is part-spherical and, as a practicalmatter, it is impossible to bend the piece 320 both longitudinally andlaterally entirely to conform to the zone 246. Instead, the piece 320 isbent only longitudinally to conform to the zone 246 in that direction,and the piece 320 is left essentially flat in the lateral direction. Thefact that the pieces 320 are relatively narrow results in the ovef-alllateral surface approximating a spherical curvature. Adhesive is appliedto the rear surface of the piece 320 and to the zone of the associatedsegment 241. A press similar to the press 282 is used to hold the piece320 until it is secured. Since the piece 320 was cut oversize, its edges321 overhang the associated zone, which edges 321 are then ground, sothat the piece 320 has precisely the desired shape.

A similar proces is performed in respect to each piece 320 and eachsegment 241. The segments are then reassembled, as shown in FIG. 49. Theouter surface of each piece 320 thereby becomes a continuation of theouter surface of the adjacent piece 320, thereby forming a configuratedsurface 323 having a multiplicity of cube corner projections thereof.

The exposed surface 323 is made conductive by depositing thereon ametallic medium by vacuum metallizing or other process. After the unitshown in FIG. 49 is suitably masked, it is inserted in the plating tankto plate a quantity of nickel or other material onto the surface 323.After a suitable thickness of nickel has been built up, the unit shownin FIG. 49 may be removed, the front surface of the build-up part maskedand then returned to the plating tank for back up. The completed moldmember 330 is shown in FIG. 50. The mold member 330 has a configuratedfront surface 331 consisting of a multiplicity of cube-corner cavities.The mold member 290 has a pair of side edges 332 and 333 respectivelylying in planes containing the center of curvature of the surface 331.The mold member 330 further has a pair of end edges 334 and 335respectively lying in chordal planes which are disposed parallel to eachother and do not pass through the center of curvature of the surface331. The front surface 331 is divided into a plurality of zones 336,separated by boundarylines 337. The planes containing the boundary lines337 are disposed parallel to-one another and do not pass through thecenter of curvature of the surface 331. The boundary lines 337 resultfrom the slight discontinuities from the various pieces 320 that make upthe surface 323..Of course, each zone 336 is spherically curved in thelongitudinal direction but is flattened in the lateral direction sincethat is the basic shape of the pieces 320 which make up the surface 323.A hole 338 is formed in the mold member 330, which hole 338 forms thefingers 200 and 201 in the finished product. Suitable projections may bemounted at the end edge 335 to enable formation of the holes 198 and 199in the completed product. I Turning now to FIGS. 51-54, there will beexplained a second alternative process for making a curved reflectorincorporating therein the features of the present invention. Thisparticular embodiment is useful in making a reflector which is curved inbut one direction as opposed to a spherical reflector which is curved inmore than one direction. FIG. 51 illustrates a cylinder 340 and asegment 341 thereof, having a partcylindrical surface 342. A pluralityof pins, such as those shown in FIG. 28, are arranged in an array, aspreviously explained. The array is placed in a plating tank to enable aquantity of nickel or the like to be deposited or electroformed onto thecube-corner projections 259 of the pins 258. The electroformingoperation is continued until a thickness of, for example, 0.007 inch isobtained. The array of pins 258 and the electroform secured thereto arethen withdrawn from the tank and an epoxy or the like is applied to theelectroform so as to retain its shape while it is being pried off thepins 258. The electroform and epoxy cast thereon are then removed fromthe pins. The metal member 345 thusly formed has a rear surface 346 anda front surface 347, which front surface 347 has a multiplicity ofcubecorner cavities therein, having the general arrangement shown inFIG. 27. The metal member 345 is then placed with the surface 347 on thepart-cylindrical surface 342 and bent to conform to the shape thereof.The member 345 is placed into the plating tank to apply additionalnickel onto the surface 346 to back up the member 345. Referring toFlG.53, the mold 350 thusly formed has a surface 347 with a multiplicity ofcubecorner cavities therein. The mold 350 is then placed in a moldingmachine into which is applied acrylic. The reflector 360 thus formed isshown in FIG. 54, having a smooth front surface 361 and a configuratedrear surface 362 with a multiplicity of cube-comer projections thereon.Suitable studs or the like may be provided in the mold for forming holes363 and 364. The reflector 360, as shown in FIG. 54, has an angularextent of 60, although any desirable size may be provided. Light from alight source, such as, for example, an elongated fluorescent lightdisposed along the axis of the cylindrical reflector 360 will strike thereflector 360 and be retrodirectively returned to the source.

It is believed that the invention, its mode of construction andassembly, and many of its advantages should be readily understood fromthe foregoing without further description, and it should also bemanifest that, while the preferred embodiment of the invention has beenshown and described for illustrative purposes, the structural detailsare, nevertheless, capable of wide variation within the purview of theinvention, as defined in the appended claims.

What is claimed is:

1. A method of making a mold for use in producing a curved reflectorhaving cube-comer elements therein, said method comprising making asubstantially-flat plate-like member having a multiplicity of cubecornerelements therein, providing a form having a curved surface with thedesired final shape of the reflector, bending said member against saidform so that the member essentially assumes the shape of the curvedsurface, forming a mold from said curved member.

2. The method set forth in claim 1, wherein said member is made ofplastic.

3. The method set forth in claim 1, wherein said member is made ofplastic having a substantially-flat rear surface and a front surfacehaving cube-corner elements therein, said rear surface being ground torender the plate-like member sufficiently thin to be bendable againstthe form.

4. The method set forth in claim 1, wherein said plate-like member isconstructed of metal.

5. The method set forth in claim 1, wherein the surface generated onsaid form is substantially smooth.

6. The method set forth in claim 1, wherein said form is provided with apart-spherical surface.

7. The method set forth in claim 1, wherein said form is provided with apart-cylindrical surface.

8. The method set forth in claim 1, wherein said member is bent againstsaid form with the cube-corner elements engaging the curved surface. I

9. The method set forth in claim 1, wherein said member is bent againstsaid form with the cube-corner elements directed away from the curvedsurface.

10. The method set forth in claim 1, wherein the mold is formed fromsaid curved surface by applying a backing to said bent member.

11. The method set forth in claim 1, wherein the mold is formed fromsaid curved surface by electroforming against said bent member.

12. A method for making a mold for use in producing a curved reflectorhaving cube-corner elements therein, said method comprising the steps ofmaking a substantially-flat plate-like member having a multiplicity ofcube-corner elements therein, providing a form having a form surface ofa selected curvature, cutting said form into a plurality of segmentseach having a portion of said form surface, cutting said member into aplurality of pieces respectively having shapes of said form surfaceportions, bending said pieces respectively onto the associated formsurface portions, and forming a mold from said bent pieces.

13. The method set forth in claim 12, wherein said form surface is partspherical, whereby the apexes of the cube-corner cavities in said molddefine a partspherical surface.

14. The method set forth in claim 12, wherein the sides of each of saidsegments are respectively defined by substantially parallel surfaces.

15. The method set forth in claim 12, wherein the arc lengths of saidform surface portions measured in a plane substantially normal to thedirection in which said segments are cut are substantially equal.

16. The method set forth in claim 12, wherein said member is cut into aplurality of pieces respectively slightly larger than the correspondingform surface portions, the margin of each of said pieces then beingground to conform precisely to the shape of the corresponding formsurface.

17. A method of making a mold for use in producing a curved reflectorhaving cube-corner elements therein, said method comprising making asubstantially-flat plate-like member having a multiplicity of cubecornerelements therein, providing a form having a form surface of a selectedcurvature, cutting said form into a plurality of segments each having aportion of said form surface, applying a flexible material to each ofsaid form surface portions, cutting said flexible material to providelengths of flexible material conforming to the shape respectively of theform surface portions, placing said lengths of flexible materialrespectively on different regions of said member, cutting said memberaround the lengths of flexible material thereon to provide a pluralityof pieces respectively having shapes of 7 said form surface portions,bending said pieces respectively onto the associated form surfaceportions, forming a mold from said bent pieces.

18. The method for making a mold set forth in claim 17, wherein saidflexible material is tape.

19. The method for making a mold set forth in claim 17, wherein thesurface of said member is painted after said lengths of material havebeen applied thereto, said lengths of material then being removed, andsaid mem-

1. A method of making a mold for use in producing a curved reflectorhaving cube-corner elements therein, said method comprising making asubstantially-flat plate-like member having a multiplicity ofcube-corner elements therein, providing a form having a curved surfacewith the desired final shape of the reflector, bending said memberagainst said form so that the member essentially assumes the shape ofthe curved surface, forming a mold from said curved member.
 2. ThemEthod set forth in claim 1, wherein said member is made of plastic. 3.The method set forth in claim 1, wherein said member is made of plastichaving a substantially-flat rear surface and a front surface havingcube-corner elements therein, said rear surface being ground to renderthe plate-like member sufficiently thin to be bendable against the form.4. The method set forth in claim 1, wherein said plate-like member isconstructed of metal.
 5. The method set forth in claim 1, wherein thesurface generated on said form is substantially smooth.
 6. The methodset forth in claim 1, wherein said form is provided with apart-spherical surface.
 7. The method set forth in claim 1, wherein saidform is provided with a part-cylindrical surface.
 8. The method setforth in claim 1, wherein said member is bent against said form with thecube-corner elements engaging the curved surface.
 9. The method setforth in claim 1, wherein said member is bent against said form with thecube-corner elements directed away from the curved surface.
 10. Themethod set forth in claim 1, wherein the mold is formed from said curvedsurface by applying a backing to said bent member.
 11. The method setforth in claim 1, wherein the mold is formed from said curved surface byelectroforming against said bent member.
 12. A method for making a moldfor use in producing a curved reflector having cube-corner elementstherein, said method comprising the steps of making a substantially-flatplate-like member having a multiplicity of cube-corner elements therein,providing a form having a form surface of a selected curvature, cuttingsaid form into a plurality of segments each having a portion of saidform surface, cutting said member into a plurality of piecesrespectively having shapes of said form surface portions, bending saidpieces respectively onto the associated form surface portions, andforming a mold from said bent pieces.
 13. The method set forth in claim12, wherein said form surface is part spherical, whereby the apexes ofthe cube-corner cavities in said mold define a part-spherical surface.14. The method set forth in claim 12, wherein the sides of each of saidsegments are respectively defined by substantially parallel surfaces.15. The method set forth in claim 12, wherein the arc lengths of saidform surface portions measured in a plane substantially normal to thedirection in which said segments are cut are substantially equal. 16.The method set forth in claim 12, wherein said member is cut into aplurality of pieces respectively slightly larger than the correspondingform surface portions, the margin of each of said pieces then beingground to conform precisely to the shape of the corresponding formsurface.
 17. A method of making a mold for use in producing a curvedreflector having cube-corner elements therein, said method comprisingmaking a substantially-flat plate-like member having a multiplicity ofcube-corner elements therein, providing a form having a form surface ofa selected curvature, cutting said form into a plurality of segmentseach having a portion of said form surface, applying a flexible materialto each of said form surface portions, cutting said flexible material toprovide lengths of flexible material conforming to the shaperespectively of the form surface portions, placing said lengths offlexible material respectively on different regions of said member,cutting said member around the lengths of flexible material thereon toprovide a plurality of pieces respectively having shapes of said formsurface portions, bending said pieces respectively onto the associatedform surface portions, forming a mold from said bent pieces.
 18. Themethod for making a mold set forth in claim 17, wherein said flexiblematerial is tape.
 19. The method for making a mold set forth in claim17, wherein the surface of said member is painted after said lengths ofmaterial have been applied thereto, said lengths of material then beingremoved, aNd said member being cut adjacent to the boundaries betweenthe painted and nonpainted areas.
 20. A method of making a mold for usein producing a curved reflector having cube-corner elements therein,said method comprising the steps of providing a die having amultiplicity of cube-corner elements respectively with apexes lyingsubstantially in a plane, electroforming against said die to provide ametal member having a multiplicity of cube-corner elements therein,removing said member from said die, providing a form having a formsurface of a selected curvature, cutting said form into a pluraity ofsegments each having a portion of said form surface, cutting said memberinto a plurality of pieces respectively having shapes of said formsurface portions, bending said pieces respectively onto the associatedform surface portions, and forming a mold from said bent pieces.
 21. Amethod of making a mold for use in producing a curved reflector havingcube-corner elements therein, said method comprising making asubstantially-flat plate-like member having a multiplicity ofcube-corner elements therein, providing a form having a form surface ofa selected curvature, cutting said form into a plurality of segmentseach having a portion of said form surface, cutting said member into aplurality of pieces respectively having shapes of said form surfaceportions, bending said pieces respectively onto the associated formsurface portions, securing said pieces respectively to the associatedform surface portions, securing said segments together so that saidpieces present a surface with a multiplicity of cube-corner elementsrespectively with apexes defining a surface corresponding substantiallyto said form surface, forming against said last-mentioned cube-cornerelements a mold having a multiplicity of cube-corner elements thereinrespectively with apexes defining a surface corresponding to said formsurface.
 22. The method for making a mold set forth in claim 21, andfurther comprising making a body having an engagement surface thatapproximately mates with one of said form surface portions, securingsaid pieces by placing adhesive on the surface thereof without saidprojections, placing said piece on the associated form surface portion,placing said body on said piece with said engagement surface on thesurface of said piece having said projections, and pressing said bodytoward said segment.
 23. The method set forth in claim 21, wherein thecube-corner elements in said member are cube-corner projections, wherebythe cube-corner elements on said mold are cube-corner projections. 24.The method set forth in claim 21, wherein said cube-corner elements onsaid pieces are projections directed away from said form surface. 25.The method for making a mold set forth in claim 21, wherein said memberis removed from said die by first applying epoxy and then prying saidmetal member from said die.
 26. The method set forth in claim 21,wherein said member is removed from said die by first applying epoxy andthen prying said metal member from said die, removing said epoxy aftersaid member has been cut into said plurality of pieces.
 27. A method ofmaking a mold for use in producing a curved reflector having cube-cornerelements therein, said method comprising the steps of providing a diehaving a multiplicity of cube-corner elements respectively with apexeslying substantially in a plane, molding plastic against said die toprovide a plastic member having a multiplicity of cube-corner elementstherein, removing said member from said die, providing a form having aform surface of a selected curvature, cutting said form into a pluralityof segments each having a portion of said form surface, cutting saidmember into a plurality of pieces respectively having shapes of saidform surface portions, bending said pieces respectively onto theassociated form surface portions, securing said segments together sothat said pieces present a surface with a multiplicity of cube-coRnerelements respectively with apexes defining a surface correspondingsubstantially to said form surface, applying a conductive coating tosaid last-mentioned surface, electroforming against said conductivecoating to provide a mold having a multiplicity of cube-corner elementstherein respectively with apexes defining a surface corresponding tosaid form surface.
 28. The method for making a mold set forth in claim27, wherein grinding the surface of said molded member opposite saidcube-corner elements to render said member substantially pliable.